THORNICROFT DRAIN EROSION ASSESSMENT · 3.1 Rapid Field Assessment In order to assess existing...

THORNICROFT DRAIN EROSION ASSESSMENT 3080 BOSTWICK ROAD, LONDON

Report Prepared for: YORK DEVELOPMENTS

Prepared by: PARISH AQUATIC SERVICES, A DIVISION OF MATRIX SOLUTIONS INC.

May 2016 Mississauga, Ontario

Suite 200, 2500 Meadowpine Boulevard Mississauga, Ontario, Canada L5N 6C4 Phone: 905.877.9531 Fax: 905.877.4143 www.parishgeomorphic.com

ii PARISH Aquatic Services

A Division of Matrix Solutions Inc.

THORNICROFT DRAIN EROSION THRESHOLD

Report prepared for York Developments, May 2016

reviewed by

Thomas Giordano, M.Sc. John Parish, P.Geo Fluvial Geomorphology Specialist Principal Geomorphologist

DISCLAIMER

We certify that this report is accurate and complete and accords with the information available during the site investigation. Information obtained during the site investigation or provided by third parties is believed to be accurate but is not guaranteed. We have exercised reasonable skill, care, and diligence in assessing the information obtained during the preparation of this report.

This report was prepared for York Developments. The report may not be relied upon by any other person or entity without our written consent and that of York Developments. Any uses of this report by a third party, or any reliance on decisions made based on it, are the responsibility of that party. We are not responsible for damages or injuries incurred by any third party, as a result of decisions made or actions taken based on this report.

iii PARISH Aquatic Services


TABLE OF CONTENTS

1 INTRODUCTION ................................................................................................................................ 1

1.1 Aims and Objectives............................................................................................................ 1

1.2 Study Area ........................................................................................................................... 1

2 BACKGORUND REVIEW .................................................................................................................... 3

2.1 Physiography and Surficial Geology .................................................................................... 3

2.2 Background Reports ............................................................................................................ 3

3 METHODOLOGY ............................................................................................................................... 4

3.1 Rapid Field Assessment....................................................................................................... 4

3.1.1 Rapid Geomorphic Assessment ............................................................................. 4

3.1.2 Rapid Stream Assessment Technique .................................................................... 5

3.2 Detailed Geomorphic Field Survey ..................................................................................... 5

4 EXISTING CONDITIONS ..................................................................................................................... 6

4.1 Reach Characterization ....................................................................................................... 6

4.2 Detailed Survey Results ...................................................................................................... 7

4.3 Instream Hydraulics ............................................................................................................ 8

5 EROSION THRESHOLD ANALYSIS ...................................................................................................... 9

6 RECOMMENDATIONS .................................................................................................................... 11

7 SUMMARY AND CONCLUSION ....................................................................................................... 12

8 REFERENCES ................................................................................................................................... 14

LIST OF FIGURES

Figure 1.1 Study Area Map .................................................................................................................. 2 Figure 4.1 Longitudinal Profile of Study Reach TD-R1 ......................................................................... 8 Figure 5.1 Permissible Unit Tractive Force for Canals in Cohesive Materials from Chow (1959) ..... 10

LIST OF TABLES

Table 3.1 Rapid Geomorphic Assessment Classification .................................................................... 5 Table 4.1 Summary of Rapid Geomorphic Assessment Results for Reach TD-R1 .............................. 7 Table 4.2 Summary of Rapid Stream Assessment Technique Results for Reach TD-R1 ..................... 7 Table 4.3 Summary of Bankfull Channel Characteristics and Instream Hydraulics ............................ 8 Table 5.1 Summary of Erosion Threshold Analysis ........................................................................... 10

APPENDICES

APPENDIX A Detailed Geomorphic Survey Summary

APPENDIX B Site Photographs

1 PARISH Aquatic Services


1 INTRODUCTION

PARISH Aquatic Services, a division of Matrix Solutions (Matrix), was retained by York Developments to

conduct an erosion assessment along a section of Thornicroft Drain in London, Ontario. To mitigate

hydrological impacts related to urban development, a preliminary Stormwater Management (SWM)

strategy has been developed for the proposed development at 3080 Bostwick Road, London. The

preliminary SWM plan intends on discharging flows into the north-south flowing channel bisecting the

development property, referred to in this document as Thornicroft Drain. This erosion assessment aims

to evaluate the receiving watercourse’s sensitivity to erosion and define a geomorphic threshold target

to be integrated into the functional design of stormwater controls.

1.1 Aims and Objectives

The objective of this geomorphic investigation is to assess and document the existing geomorphic

condition of a section of Thornicroft Drain – the receiving channel – and quantify hydraulic parameters

influencing channel form and function to aid in the development of an effective SWM plan. In support of

this assessment, the following tasks were undertaken:

Review of relevant background information pertaining to the proposed 3080 Bostwick Road development and Thornicroft Drain study area;

Conduct rapid field assessments and a detailed field survey to quantify existing channel dimensions and evaluate the geomorphic condition of the watercourse within the study area;

Evaluate collected field data to determine instream hydraulics and define appropriate erosion threshold limits for effective stormwater management;

Provide recommendations from a geomorphic perspective based on the SWM plan developed by IBI (2015).

1.2 Study Area

The proposed development is located at 3080 Bostwick Road in London, Ontario. This study focuses on

an open channel flowing north to south, bisecting the 3080 Bostwick Road property. This watercourse is

part of the Dingman Creek watershed and has been referred to as Tributary ‘D’ in the Dingman Creek

Subwatershed Study Update (Delcan, 2005) but referred to as Thornicroft Drain in this document. The

area of interest is located in the headwaters of the tributary south of Southdale Road West and east of

Bostwick Road. A stormwater outfall discharging to the south of Southdale Road West at the north end

of the development property is considered the source of Thornicroft Drain Tributary. The subject

property is currently vacant, although bounded by residential developments to the north, agricultural

land to the west and south, and some commercial land to the east. A narrow riparian corridor



comprised of deciduous trees and sparse shrubs boarders the channel at south end of the proposed

development area.

Due to the time sensitive nature of the project and the onset of winter conditions limiting availability of

suitable field work conditions, the current geomorphic investigation focuses on the proposed outlet

location. A more comprehensive geomorphic assessment should be completed to characterize channel

dimensions and assess stability of the entire tributary in order to properly assess potential risk to

downstream channel areas. Of notable consideration are two Environmentally Significant Areas (ESA)

identified south of the subject property in Schedule B1 of the City of London’s Official Plan. The woodlot

area located immediately downstream of the subject property has been designated as a “Significant

Woodland” area. A “Significant Wetland” feature has also been designated further downstream along

Thornicroft Drain prior to its confluence with Dingman Creek approximately 2.5km south of the subject

property. A thorough understanding of the geomorphic character and hydraulic parameters which

govern channel form and function is essential to set meaningful threshold limits and minimize potential

effects of development within the Thronicroft Drain headwater areas.

Figure 1.1 Study Area Map



2 BACKGORUND REVIEW

2.1 Physiography and Surficial Geology

The subject property is located at the west end of the Mount Elgin Ridges physiographic region. This

physiographic region consists of a till plain landform that stretches from the Thames Valley and Norfork

sand plains characterized by a succession of clay or silt clay ridges and vales of gravel, sand or silt

alluvium (Chapman and Putnam, 1984; 2007). The surficial geology has been characterized as clay to silt

textured till likely derived from glaciolacustrine deposits (Chapman and Putnam, 1984; 2007). More

recent geotechnical investigations support these findings as the predominant soil type within study area

was described as fairly impermeable clay to silt textured till with trace sand inclusions (IBI Group, 2015;

LVM, 2014). The fine textured nature characteristic of the study areas geological setting can have

significant influence on the watercourses sediment load and sensitivity to erosion.

2.2 Background Reports

IBI Group. 2015. 3080 Bostwick Road Stormwater Drainage and Stormwater Management Plan.

Prepared for York Developments. December 2015.

This report prepared by IBI Group (IBI) outlines the preliminary SWM plan for the proposed

development at 3080 Bostwick Road. The preliminary report aims to address potential impacts of

increased surface water runoff, quality of runoff and erosion control through the implementation of the

SWM plan presented. The proposed SWM plan encompasses the entire property, but focuses on the

northern two thirds of the property to the east of the open channel in detail while the remainder of the

site is still at a conceptual stage. IBI has examined the feasibility of providing quantity and quality

controls to determine potential effects on the existing channel.

On the east side of the open channel, a storm sewer is to be constructed on the north-south and the

future east-west streets to convey minor flows to the existing open channel running through the center

of the property. Maintaining an open channel was determined to be the preferred alternative to convey

flows to avoid costly piping alternative or fill options. In the future another storm sewer will be

constructed on the east-west street to collect minor flows from the west side of the open channel.

Future development on each of the proposed Blocks will be connected by storm pdc’s to these storm

sewers. In the interim, until the road crossing of the creek is constructed, the storm sewer is to

discharge into the open channel via a temporary outlet with erosion protection. Ultimately the storm

sewer will connect into future culverts at the road crossing which will convey the open channel flows

from the north. A scoped EIS to support the outlet works is to be submitted under a separate cover.

Major flows from the proposed Blocks are to be directed to via proposed street pattern and grading

conveying major overland flows to the open channel. The report suggests that increases in anticipated

runoff and decreases in infiltration would be alleviated through the incorporation Best Management



Practices where possible. The SWM plan proposes controlling flow quantities by restricting post-

development flows to pre-development levels through surface storage, roof storage, and underground

storage where possible. Quality controls are to be provided through the installation of oil/grit

separators. The importance of implementing proper erosion and sediment control (ESC) measures were

reinforced and planned ESC measures were discussed in detail.

LVM Inc. 2014. Preliminary Geotechnical Test Pit Investigation 3080 Bostwick Road. Submitted to York

Development Group, June 6, 2014.

LVM Inc. (LVM) performed a preliminary geotechnical investigation at 3080 Bostwick Road to determine

subsurface conditions at the site. Twelve (12) test pits (TP) were excavated to depths of 3.05m to 3.6m

to document soil stratigraphy and determine natural moisture content. Subsurface conditions were

generally characterized as consisting of a stiff, grey to brown silty clay till with some sand inclusions.

The report provided preliminary geotechnical recommendations for the design of foundations, services

and pavements for the 3080 Bostwick Road development.

3 METHODOLOGY

Matrix personnel were required to comply with legislated, Matrix, and York Developments health and

safety standards. Matrix staff conducted both rapid field assessments and a detailed field survey on

January 11, 2016 to gain a general understanding of the geomorphic processes operating within the

system, evaluate channel stability, and collect detailed data to characterize bed substrate and quantify

channel form. A summary of field investigations and protocols adhered to is presented in the

subsequent sections.

3.1 Rapid Field Assessment

In order to assess existing geomorphic conditions and document any evidence of channel instability, a

rapid field assessment was undertaken along the watercourse adhering to two rapid field assessment

protocols – Rapid Geomorphic Assessment (RGA) and Rapid Stream Assessment Technique (RSAT).

These assessments offer a quick qualitative evaluation of the watercourse providing insight into active

geomorphic processes and overall channel stability, as well as an estimate of the ecological integrity.

Results of the rapid field assessments are summarized in Section 4.1.

3.1.1 Rapid Geomorphic Assessment

An RGA documents observed indicators of channel instability within a watercourse to assess its existing

geomorphic stability and sensitivity to an alteration in the sediment-flow regime (MOE, 2003).

Observations are quantified using a stability index that identifies channel sensitivity based on evidence

of channel widening, degradation, aggradation, and planimetric form adjustment. The RGA protocol

produces a stability index value which classifies the channel as being in regime (score <0.20),

stressed/transitional (score 0.21 to 0.40), or in adjustment (score >0.41). The various classifications



(Table 3.1) indicate whether evidence of instability observed is isolated and/or the result of normal river

function or wide spread and resulting from a change in the sediment-flow regime of the system.

Table 3.1 Rapid Geomorphic Assessment Classification

Factor Value Classification Interpretation

≤0.20 In Regime or Stable (Least Sensitive)

The channel morphology is within a range of variance for streams of similar hydrographic characteristics – evidence of instability is isolated or associated with normal river meander propagation processes

0.21-0.40 Transitional or Stressed (Moderately Sensitive)

Channel morphology is within the range of variance for streams of similar hydrographic characteristics but the evidence of instability is frequent

≥0.41 In Adjustment (Most Sensitive)

Channel morphology is not within the range of variance and evidence of instability is wide spread

3.1.2 Rapid Stream Assessment Technique

The RSAT provides a broader view of the system by considering the ecological function of the stream

through observations of instream habitat, water quality, riparian conditions, and biological indicators

(Galli, 1996). The RSAT approach integrates semi quantitative measures of bankfull channel dimensions,

type of substrate, vegetative cover, and channel disturbance to provide a broader assessment of the

overall health and functions of a river reach. Based on the observed channel conditions, RSAT scores

rank the channel as maintaining a low (<20), moderate (20 to 35), or high (>35) degree of stream

ecological health.

3.2 Detailed Geomorphic Field Survey

To quantify the geomorphic characteristics of the study reach and evaluate instream hydraulics, a

detailed survey was conducted following standard protocols that utilize known field indicators to

quantify the bankfull cross-sectional dimensions of the selected reach. Five cross-sections were

measured within a 180m stretch of channel located downstream from the proposed SWM outlet

location. The detailed investigation characterized channel bed and subsurface materials at each cross-

section using a modified Wolman pebble count (Wolman, 1954) for coarse material and a textural

ribbon technique for fine materials. Channel bank properties (i.e. form, vegetative cover, and materials)

were also recorded at each cross-section to obtain a comprehensive understanding of channel boundary

conditions. A longitudinal profile survey was completed using a total station to capture the channel bed

gradient and determine the watercourse energy gradient from identified bankfull locations at each

cross-section. Results of the detailed survey are summarized in Section 4.2 and details are included in

Appendix A.



4 EXISTING CONDITIONS

4.1 Reach Characterization

The main flows conveyed through study reach are derived from the urbanized area to the north being as

stormwater flows are discharge through an outlet located at Southdale Road West. The channel flows

through a vacant lot previously utilized as an agricultural field which is soon to be developed. With

regards to channel form, a low flow channel entrenched within a slightly broader, sparely vegetated

high flow conveyance channel was characteristic of the reach. The watercourse demonstrates a more or

less straight planform configuration exhibiting a low channel gradient as evidenced through slow

moving, almost stagnant water at the time of visit. The channel displayed little to no variability in bed

morphology with an absence of riffle features. Any form of bed variability was attributed to fine

textured sediment deposits and localized bed scour. A narrow riparian corridor comprised of deciduous

trees and spare shrubs lines the channel and becomes more prevalent in the downstream direction.

Areas where trees were situated at the water-bank interface often resulted in exposed roots and

leaning/fallen trees, although this was more common upstream than downstream.

The channel appears to be exhibiting minor signs of stress attributed to stormwater flows released

upstream. The high velocity flows discharged from the stormwater outlet exert high erosive energy

along bed and banks when discharged. As a result, systemic deposition of eroded material ensues in the

downstream direction as transport capacity is reduced due to waning flow velocities with distance from

the outlet. The coarser materials, predominantly sands, are deposited closer to the source forming bar

deposits in the upstream section of the study reach. Finer materials travel further downstream as

suspended load causing extensive siltation with high organic content further downstream.

Results of the RGA indicate the channel is in a state of transition receiving a score of 0.24 (Table 4.1).

Aggradation and widening were determined to be the primary processes operating within the study

reach. This was evidenced through occurrence of lateral sediment deposits, extensive siltation, and

exposed tree roots. Evidence of channel form adjustment and degradation were also apparent as fine

sediment deposits were poorly formed/reworked, and areas of exposed underlying till were recorded.

An RSAT score of 19 classifies the reach as being of low ecological health (Table 4.2). This is

predominantly due to the lack of habitat variability in the channel, high degree of fine sediment

suspended in the water column and absence of biological indicators at the time of visit.



Table 4.1 Summary of Rapid Geomorphic Assessment Results for Reach TD-R1

RGA SUMMARY

Reach

Factor Value

Stability Index Condition Aggradation Degradation Widening

Planimetric Adjustment

TD-R1 0.36 0.14 0.29 0.14 0.23 Transitional

Table 4.2 Summary of Rapid Stream Assessment Technique Results for Reach TD-R1

RSAT SUMMARY

Reach

Factor Value Overall Score

Condition Channel Stability

Scour/ Deposition

Instream Habitat

Water Quality

Riparian Condition

Biological Indicators

Max. Score

11 8 8 8 7 8 50

TD-R1 4 3 2 3 4 3 19 Low

4.2 Detailed Survey Results

The low flow channel was identified as the bankfull cross-section as major bank inflection points and

changes in vegetation delineated the elevation of recurring flows which govern channel form and

function within the reach. Results of the detailed survey determined that bankfull channel widths were

fairly consistent ranging from 3.97m to 5.07m with an average width of 4.49m. Bankfull depths varied

from 0.39m to 0.55m with an average depth of 0.46m. Channel bed substrate was typically comprised of

fine textured materials, predominantly consisting of medium sand, fine sand and silt with trace pebbles

and fine gravel inclusions. The boundary material along the bed of the channel generally displayed a

transition from soft, unconsolidated material with high organic content to a more compact underlying

till. The subsurface till material was identified as a loose to fairly compact silty clay till with considerable

sand inclusions and displaying low to moderate plasticity. The substrate distribution of the soft,

unconsolidated overlaying materials exhibited a reach average D50 of 0.17mm (fine sand), D84 of 1.34mm

(coarse sand) and D95 of 5.70mm (Pebble). The longitudinal profile presented in Figure 4.1 illustrates

channel bed morphology and energy gradient characteristic of the survey reach. The channel displayed a

low energy gradient of 0.13% with minor bed variability resulting from localized bed scour and fine

textured sediment deposits.



Figure 4.1 Longitudinal Profile of Study Reach TD-R1

Table 4.3 Summary of Bankfull Channel Characteristics and Instream Hydraulics

Parameter TD-R1

Average Bankfull Width (m) 4.49

Average Bankfull Depth (m) 0.46

Average Maximum Bankfull Depth (m) 0.68

Energy Gradient (%) 0.13

Manning’s Roughness Coefficient ‘n’ 0.03

D50 (mm) 0.17

D84 (mm) 1.34

D95 (mm) 5.71

Bank Materials si/fs/ms

Bankfull Discharge (m3/s) 1.64

Average Bankfull Velocity (m/s) 0.65

Average Shear Stress (N/m2) 5.32

Maximum Shear Stress (N/m2) 8.47

4.3 Instream Hydraulics

Using parameters established from the detailed survey, instream channel hydraulics under the field

identified bankfull conditions were evaluated. The detailed field assessment provides preliminary



parameters that characterize channel geometry, substrate, and energy gradient which are then applied

for the evaluation of hydraulic properties specific to the site such as, bankfull discharge, flow velocities

and shear stress among others (Table 4.3). To properly evaluate instream hydraulics, a Manning’s ‘n’

value of 0.030 was applied in the assessment of hydraulic parameters as it was deemed appropriate to

represent observed channel roughness conditions. An average bankfull discharge of 1.64m3/s was

determined to govern geomorphic form and function as evidenced through known field indicators of the

bankfull condition identified in the field. Under bankfull conditions, an average flow velocity of 0.65m/s

and maximum shear stress of 8.47N/m2 was evaluated.

5 EROSION THRESHOLD ANALYSIS

The critical discharge, or erosion threshold, is evaluated through a quantitative analysis of site

conditions. The threshold limit defines instream conditions during which sustained flows will initiate

particle entrainment and instigate sediment transport. The detailed geomorphic survey was undertaken

to quantify the geomorphic characteristics of the receiving channel and evaluate instream hydraulics to

define an appropriate erosion threshold limit based on critical shear stress and permissible velocities.

A permissible velocity method developed by Neill (1967) was undertaken to define the critical discharge

of the overlying, unconsolidated bed substrate. This methodology is best suited to determine

permissible velocities for fine grained materials, specifically coarse sands, which characterized the

coarser fraction (D84) of the unconsolidated materials. An iterative approach using existing channel

conditions was applied to evaluate the discharge at which the defined permissible velocity was

generated (Table 5.1). Results of the analysis determined that a critical discharge of 0.37m3/s was

appropriate to define flow conditions suitable to cause significant transport of the unconsolidated

substrate material. An average velocity of 0.44m/s – maximum of 0.56m/s – would be produced under

the critical flow condition, generated at approximately 30% of the determined bankfull discharge. It is

estimated that instream hydraulics of this nature would be generated at an average water depth of

0.24m – maximum of 0.33m – within the existing channel geometry.

Although comprised of finer materials, the cohesive nature of the channels underlying clay till requires

higher flow velocities and shear stresses to instigate transport of materials. The slightly coarser

overlying, loose materials – fine sand to coarse sand – are more readily available for transport than the

fairly compact underlying till. Due to the exposure of the underlying till material along the channel bed

and toe of banks, it is speculated that critical shear stresses for this material also influences the

geomorphic character of the study reach. In an effort to define the critical shear stress for the observed

loose to fairly compact silty clay till, Figure 7-11 from Chow (1959) was utilized to obtain an estimate of

permissible tractive force this representative of the material encountered. The table referenced defines

permissible tractive force for cohesive materials based on level of compaction and composition (Figure

5.1).



Table 5.1 Summary of Erosion Threshold Analysis

Parameter

Unconsolidated Substrate Material

Underlying Till Material

Defining Criteria Critical Shear Stress (N/m

2) -- 4.31

a

Average Permissible Velocity (m/s) 0.50b

--

Erosion Threshold Results

Critical Discharge (m3/s) 0.37 0.60

Maximum Velocity (m/s) 0.56 0.66

Maximum Depth (m) 0.33 0.41

Method Neill (1967) Chow (1959)

a Critical shear stress for loose to fairly compact sandy clays (<50% sand) defined using Fig 7-11 from Chow (1959) and converted to N/m2 b Average Permissible velocity for overlying, unconsolidated sand material evaluated through application of Neill (1967) methodology

Figure 5.1 Permissible Unit Tractive Force for Canals in Cohesive Materials from Chow (1959)

A critical shear stress of 4.31N/m2 for loose to fairly compact sandy clays (<50% sand) was considered to

best represent the underlying till material observed within the study reach. A shear stress approach that

applied the tractive force obtained for Figure 7-11 was undertaken to define a critical threshold limit for

the underlying till material. Under existing channel geometry, a discharge of 0.60m3/s would generate

hydraulic conditions that exhibit the defined shear stress. This discharge is therefore deemed



appropriate to define the critical discharge for the underlying till material (Table 5.1). Under this critical

flow condition an average velocity of 0.51m/s – maximum of 0.66m/s – would be produced equaling

approximately 38% of the bankfull discharge. It is estimated that hydraulic conditions of this nature

would be generated at an average water depth of 0.30m – maximum of 0.41m – within the existing

channel geometry.

Given the existing geomorphic condition and substrate characteristics observed, the critical discharge

for the overlying, unconsolidated sand material was deemed most suitable to define the erosion

threshold for the receiving watercourse downstream of the proposed outlet. Applying a critical

discharge of 0.37m3/s to finalize the functional SWM plan provides a conservative approach aimed at

maintaining geomorphic form and function within a stretch of channel currently experiencing minor

forms of adjustment from similar SWM practices upstream.

The defined threshold value determined through the detailed assessment of Reach TD-R1 provides a

contemporary evaluation of the geomorphological limits that govern channel form and function of the

receiving watercourse at the proposed outlet location described in the 3080 Bostwick Road SWM plan

(IBI, 2015). Erosion control along the receiving watercourse is an integral component of any SWM

functional design. To mitigate the potential risk of adverse geomorphological adjustments resulting from

direct/point source stormwater discharge within the upstream section of Thornicroft Drain, the critical

discharge should be utilized in the development of a comprehensive SWM functional design. It is

recommended that efforts to mitigate the erosive potential of stormwater flows discharged from the

outlet are made to maintain the geomorphological form and function of the receiving channel.

6 RECOMMENDATIONS

Erosion control is an integral component of the functional SWM as urban development tends to alter

natural hydrologic regimes typically resulting in significant adjustments to the geomorphological form

and function of the receiving watercourse. Application of the erosion threshold in the development of a

functional SWM plan is essential to help maintain the functional geomorphological processes operating

within the system and mitigate the impacts of urban development.

Based on the preliminary SWM plan developed by IBI (2015), the following recommendations are

presented from a geomorphic perspective:

a. A more comprehensive fluvial geomorphic assessment should be completed to characterize

geomorphic conditions, assess channel stability and identify the potential risk to the entire

tributary as a result of the proposed development. The fluvial geomorphic assessment should be

integrated into the final EIS to provide a holistic investigation of potential impacts to the existing

watercourse to assist in the development of appropriate stormwater management solutions.



b. SWM and channel alterations should aim to mitigate adverse impacts to downstream areas as

Schedule B1 of the City of London Official Plan (Natural Heritage Features) identifies two ESA’s

along the Thornicroft Drain. The woodlot area located immediately downstream of the subject

property has been designated as a “Significant Woodland” area. Further downstream a wetland

feature located near the drains confluence with Dingman Creek has been designated a

“Significant Wetland” feature. Significant land use changes within the headwater area of the

drain may alter its natural hydrologic regime enough to trigger the onset of channel adjustment.

Point source influences on the hydrologic regime can cause adverse effects to develop at a local

scale, often spreading downstream. The potential risk to downstream environments should also

be considered as channel adjustments in upstream areas will likely propagate downstream

threatening vulnerable ecosystems downstream.

c. With regards to the interim solution, discharging directly into the watercourse is not the

preferred solution, even with erosion protection established. Locating the outlet away from the

existing watercourse and constructing an outlet channel which incorporates natural instream

flow energy dissipation measures prior to entering the existing watercourse is a preferred

alternative. Nonetheless, the orientation of the proposed outlet with relation to the receiving

channel and stone size prescribed should be reviewed in detail and confirmed prior to

finalization of SWM plan. It is recommended that the proposed open channel design include a

low flow channel demonstrating a natural channel form capable of conveying more frequent,

less intense flow events. Incorporating a low flow channel within a broader high flow

conveyance channel would maintain geomorphic integrity and enhance sediment transport

during low flow events. It is recommended that alterations to the existing channel form

integrate fluvial geomorphic principles to mitigate development of adverse impacts to channel

form and function which may propagate downstream.

d. Based on the ultimate solution described, it is recommended that fluvial geomorphic principles

be considered in the future design of the road crossing over Thornicroft Drain. The design

should draw from a more comprehensive fluvial geomorphic assessment to mitigate potential

risk to downstream areas.

7 SUMMARY AND CONCLUSION

To assist with the development of a functional SWM plan for the proposed development at 3080

Bostwick Road, a geomorphic erosion assessment was undertaken to define an erosion threshold limit

for the receiving watercourse, Thornicroft Drain. Due to the time sensitive nature of the project and the

onset of winter conditions limiting availability of suitable field work conditions, the geomorphic

investigation focused specifically on the proposed outlet location.

A rapid field assessment was conducted within the study reach to assess channel stability and determine

active geomorphic processes operating within the system. Based on the rapid field assessments, the



channel appears to be exhibiting minor signs of stress attributed to stormwater flows released upstream

with stability indices suggesting it was in a “transitional” state. The high velocity flows discharged from

the stormwater outlet exert high erosive energy along bed and banks when discharged. As a result,

systemic deposition of eroded material ensues in the downstream direction concentrating near the

proposed outlet location. The predominant forms of adjustment were aggradation and widening,

although some instances of degradation and planform adjustment were also noted. A detailed

geomorphic survey was conducted to quantify bankfull channel dimensions, determine energy gradient

and characterize channel bed substrate within the study reach located directly downstream of the

proposed outlet location. Detailed survey results were used to evaluate instream hydraulics and define a

critical discharge for sediment mobility. A critical discharge of 0.37m3/s was determined to

appropriately define sediment transport threshold limits under existing channel conditions.

Based on our understanding of the geomorphic condition of the study reach, the site’s sensitivity to

erosion is largely a result of on-going issues derived from SWM practices upstream. Although offsetting

the outlet away from the channel is preferred from a geomorphic perspective, the interim solution

presented in the 3080 Bostwick Road SWM plan (IBI Group, 2015) may reduce the potential risk of

erosion at the proposed outlet location. However, it should be noted that localized erosion control will

not mitigate the on-going issues affecting the receiving watercourse at its source. Future large scale

remediation work along Thornicroft Drain is anticipated to aid in accommodating the receiving

watercourses’ altered hydrologic regime as a result of land use changes within its headwaters. A

comprehensive fluvial geomorphic investigation of the entire tributary is recommended to assess the

geomorphic character and systemic processes operating within the tributary to properly assess potential

risk to downstream areas and develop responsible long-term solutions relating to urban development

and SWM.



8 REFERENCES

Chapman, L.J. and Putnam, D.F. 1984. Physiography of Southern Ontario. Ontario Geological Survey

Special Volume 2. 270p.

Chapman, L.J. and Putnam, D.F. 2007. Physiography of Southern Ontario: Ontario Geological Survey,

Miscellaneous Release – Data 228.

Chow V.T. 1959. Open-Channel Hydraulics. New York, NY: McGraw-Hill Book Co.

Delcan Corporation. 2005. Dingman Creek Subwatershed Study Update (DCSSU). Prepared for the City

of London. April 2005.

Galli J. 1996. Rapid Stream Assessment Technique, Field Methods. Metropolitan Washington Council of

Governments. Washington, DC. 36pp.

IBI Group. 2015. 3080 Bostwick Road Stormwater Drainage and Stormwater Management Plan.

Prepared for York Developments. December 2015.

LVM Inc. 2014. Preliminary Geotechnical Test Pit Investigation 3080 Bostwick Road. Prepared for York

Development Group, June 6, 2014.

Ontario Ministry of the Environment (MOE). 2003. Stormwater Management Planning and Design

Manual. Queen’s Printer. Ottawa, Ontario. March 2003.

http://www.ontario.ca/document/stormwater-management-planning-and-design-manual

Wolman M. G. 1954. A Method of Sampling Coarse River-Bed Material. Transactions, American

Geophysical Union 35 (6): 951-956.

http://www.ontario.ca/document/stormwater-management-planning-and-design-manual

APPENDIX A Detailed Geomorphic Survey Summary

Project: Thornicroft Drain Erosion Assessment

23059-522-146523

Site Location: 3080 Bostwick Road, London

Reach: TD-1

Length surveyed: 177.36 m

Number of cross-sections: 5

Date of Survey: 11-Jan-16

Modifying Factors

Surrounding Land Use: Agricultural field, deciduous woodlot

General Riparian Vegetation: Scattered Deciduous trees, sparse shrubs, grasses

Existing Channel Disturbances: Stormwater outlet at upstream end of reach

Woody Debris: minor occurences of woody debris found within channel

Cross-Sectional Characteristics

Average

Bankfull Width (m) 3.90 - 4.95 4.16

Bankfull Depth (m) 0.40 - 0.54 0.49

Width / Depth 7.63 - 9.94 8.56

Wetted Width (m) 2.36 - 3.37 2.87

Water Depth (m) 0.07 - 0.24 0.16

Wetted Width / Depth 11.25 - 32.67 22.21

Manning's n 0.03

Range

DETAILED GEOMORPHOLOGICAL FIELD DATA SUMMARY

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Bank Characteristics

Average

Bank Height (m) 1.7 - 2.5 2.08

Bank Angle (degrees) 20 - 55 33

Root Depth (cm) 7.0 - 15 10.0

Root Density (1=Low - 5=High) 1 - 2 1.5

Protected by vegetation (%) 70 - 80 75.0

Amount of undercut (cm) No Undercutting Observed

Bank Materials: Torvane values (kg/cm2)

si/fs/ms * N/A due to frozen banks

si/vfs/fs

si/s/cs

* - Dominant Material

Profile Characteristics

Bankfull Gradient: 0.13 %

Bed Gradient: 0.20 %

Range

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Substrate Characteristics

Bed Substrate Description:

Loose substrate comprised of fine to coarse sand particles with trace pebble and small gravel inclusions

Bed material became finer in downstream direction and darker in colour (dark brown to black) due to increasing

organic content

Sub-pavement Description:

Consisted of loose to fairly compact silty clay till with sand inclusions.

Underlying material has become exposed in some areas due to localized scour, but typically found below

loose sandy material

Particle Sizes (cm):

Pebble Counts Description

D10 0.006 cm very fine sand to silt

D50 0.017 cm fine sand to medium sand

D95 0.57 cm pebbles to fine gravel

0

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Particle Size (cm)

Substrate Size Distribution

Total %

Cumulative %

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Field Observations

Low flow channel entrenched within a broader, high flow conveyance channel (15m wide, 1.8m deep). Minor

signs of basal scour observed throught reach. Areas where vegetation/dead woody debris was found along the

low flow channel boundary typically coincided with localized bed/bank erosion. Sediment deposits were widespread

with the upstream area of the detailed survey section. Deposits consisted of sand materials but became noticable

finer in the downstream direction. Deposits also became less prevalent in downstream direction. Material on bed

was typically soft, unconsolidated consisting of sand to silt material. Subpavement material consisted

of loose to fairly compact silty clay till with sand inclusions. Localized exposure of underlying till material was

noted along bed and toe of banks. Organic matter content and percentage of fine material (silt) increased in

downstream direction. Cold temperatures and winter conditions left banks partially frozen with small amount of

snow cover, with thin ice forming along channel boundaie at the time of visit.

XS 1

Evidence of channel widening noted approximately 10m upstream and downstream of XS

Bed comprised of loose sand materials - medium sand

LB steep demostrating minor signs of scour along toe

Poorly formed sand deposit observed along base of LB upstream and downstream of XS

Subpavement consists of loose fine sands, silt, clay and heavy organics

XS 2

Channel bed consists of loose sand materials - medium to coarse sand (slightly coarser than XS1)

Signs of basal scour along toe of banks

Poorly formed sand sediment deposits observed along channel boundaries

Tree stumps/embedded debris noted along channel boundaries upstream and downstream of XS, appear to

interact with flow during higher flow events

XS 3

XS located downstream of transition into woodlot area south of 3080 Bostwick Road property

Channel substrate consists of loose sand material - fine sand (finer than XS 1 and XS 2)

Low flow channel continues to be entrenched within broader, High flow channel (15m wide, 2m deep)

Fallen tree spanning top of flow flow channel, not incontact with flow, approximately 1m DS of XS

Bed was soft and unconsolidated with siltation occuring - water was turbid

Exposed roots and basal scour along LB was noted

XS 4

Fallen tree located on LB at XS

Bed material was soft and unconsolidated but exposed till felt below loose material.

Loose bed material consisted of very fine sand and silt with high organic content

Watercrest was noted along LB indicating potential ground water seepage

Exposed till was observed along the toe and bed of LB

XS 5

Left and right side of channel bed consisted of fine, unconsolidated material - very fine sand, silt and organics

Compact till material exposed within center channel area

Siltation occuring within channel

Woody debris jam located approximately 20m downstream of cross-section

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APPENDIX B Site Photographs

PARISH Aquatic Services A Division of Matrix Solutions Inc.

1

APPENDIX B SITE PHOTOGRAPHS

THORNICROFT DRAIN

Matrix Supplied January 11, 2016


Photo 1: XS 1 facing downstream. Basal scour along toe of both banks. Note low flow (bankfull) channel situated within broader high flow conveyance channel. Relic tree trunks/embedded debris sporadically found along channel boundaries.

Photo 2: XS 1 facing upstream. Poorly formed sand deposits along channel banks upstream of cross-section. Otherwise, channel lacking variability in bed morphology.


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Photo 3: XS 2 facing downstream. Relic tree trunks observed along channel boundaries and influencing channel hydraulics during higher flow events.

Photo 4: Substrate examined at XS 2. Comprised of unconsolidated fine to medium sand with trace pebble and small gravel inclusions.


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Photo 5: XS 3 facing downstream. Channel transitions into deciduous woodlot area. Leaning/fallen trees noted along banks.

Photo 6: XS3 facing upstream. Water turbid with siltation occurring in channel and along channel boundaries.


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Photo 7: XS4 facing downstream. Channel continues to lack variability in bed morphology in downstream direction. Sediment deposits less prevalent than upstream.

Photo 8: XS 4 facing upstream. Basal scour noted along both banks. Scattered trees and sparse shrubs characterize the riparian area.


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THORNICROFT DRAIN



Photo 9: XS5 facing downstream. Basal scour noted along the toe of both banks. Woody debris jam visible approximately 2m downstream of XS.

Photo 10: XS5 facing upstream. Relic tree trunks continued to be sporadically observed along channel boundaries.

THORNICROFT DRAIN EROSION ASSESSMENT · 3.1 Rapid Field Assessment In order to assess existing...

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